Add About & Privacy and Donate modals (M6, §01)

src/app/App.tsx

@@ -89,6 +89,22 @@ export function App() {
         >
           Export
         </button>
+        <button
+          type="button"
+          className={styles.headerButton}
+          onClick={() => openModal('about')}
+          title="About, keyboard shortcuts, and privacy information"
+        >
+          About &amp; Privacy
+        </button>
+        <button
+          type="button"
+          className={styles.headerButton}
+          onClick={() => openModal('donate')}
+          title="Support Astrolabe"
+        >
+          Donate
+        </button>
         <ThemeToggle />
         {/* Hidden picker for Import; restricted to JSON (spec §08). */}
         <input